It's hard to come up with a perfect analogy since the motor is a pretty complicated system
Also I'm sure there are some differences to the dynamics of water vs. air, and not to mention the chain reaction that occurs with boost. The conveyor belt increases in speed as does the force behind the water as each bucket goes by

Ah yes that's true, I really wasn't even taking into consideration the amount of power produced by the engine...just airflow.

As per the two hoses analogy, you could get a slightly higher flow if you had a larger hose with a restriction at the end. But it depends on the length of hose that was used and the flowrates.

However you can't really consider the engine to be a constant restriction either. Think about when you're cruising at constant speed. Usually I find i'm at about 11mm Hg vaccuum while cruising, regardless of the engine speed, so obviously the engine can flow different amounts of air given the same intake pressure.

But this is always a good topic to chat about once in the while.
Everyone have something to say!

It's everyone's chance to feel either bedazzled by brilliance or baffled by bullshit

Sorry guys haven't had a chance to fully read everyone's posts yet, I will tonight.

Here's a reply from my cousin...I don't think he can access the site from his workplace.

Begin quote //

I misquoted when I was explaining to my cousin about larger turbos. Yes a larger turbo will flow more air than a smaller turbo at a given pressure however in an engine the power will always be proportional to the number of air molecules (assuming also you can supports its combustion) and if the intake manifold is holding 15psi to redline that means the engine is not able to “eat” the air any faster. What this means is that whatever turbo your using is adequate 14B or FP RED.

What I am trying to say is that if a small turbo can maintain pressure in the intake manifold (say 15psi) all the way to redline then a bigger turbo will not put in any more air molecules at the same pressure (ASSUMING THAT EVERYTHING ELSE IS CONSTANT LIKE INTAKE AIR TEMPERATURE AND EXHAUST BACKPRESSURE).

Most people think that you slap on a bigger turbo (with supporting mods) and all of a sudden you are much faster. If you slap on the bigger turbo (with supporting mods) and use the same boost pressure you will get more power but that is because the intake temperature will be lower and there will be less restriction in the exhaust (bigger turbine housing). If you heated the air to what it would have been with the smaller turbo and put a restriction into the exhaust to match the backpressure of the smaller turbo then you WILL NOT make more power with the bigger turbo (AT THE SAME PRESSURE) because the smaller turbo was not maxed out. Now do this experiment again at 25psi and then you will see a power increase because at that pressure the smaller turbo will not be able to maintain to redline whereas the bigger turbo will. Now you have extra boost pressure which you didn’t have before.

But the point of this was that the power is coming from the extra boost which was not there with the smaller turbo. If the smaller turbo can maintain that boost you want then the only benefit from a bigger turbo is increased efficiencies in the intake temperature and exhaust.

//end quote.

air temperature = 70F, density at 20psi = 0.177 lbs per cubic foot

air temperature = 150F, density at 20psi = 0.154 lbs per cubic foot

air temperature = 200F, density at 20psi = 0.142 lbs per cubic foot

air temperature = 400F, density at 20psi = 0.109 lbs per cubic foot

The higher the boost (pressure in psi) the hotter the air will be exiting the turbo. Quantity of air is determined (ignoring flow in CFM for now) by it's density which is directly related to temperature. A larger turbo can produce compressed air at a lower temperature than a smaller one.

Here is an example. You take a 16g turbo and set it at 10psi. Dyno the car from 5000rpm to 6000rpm. You then take the FP3065 and do the same. I bet the power difference is negligible. That's because they're both well within their efficiency range.

Now do it again at 25psi. At this point, the 16g's air will be MUCH hotter than the FP3065, so at the same boost, the air going in will be much denser for the FP3065, thus netting higher HP and more MASS flow of air.

Of course there is more to it than that. Engine RPM will also determine the mass air flow the turbo is required to output along with other factors. My example is to show how much temperature matters, otherwise there will be no use for a large FMIC.

As for the boost gauge issue. Mechanical boost gauges are what's called a Bourdon Tube gauge. Fluctuations/oscillations will only net an average reading which is good enough.

Ok so given that there may be a difference in temperature due to different efficiencies, I still doubt that the change in temperature due to a slight difference in efficiency would make a huge difference compared to the overall temperature change caused by actually compressing the air. So I think that the two turbos, both working around 70% efficiency will have similar outlet temperatures. I could be wrong about that though.

There are more fundamental differences in the flow with the two different compressors. For example if you take a look at the flow map for the 14B http://www.stealth316.com/images/td05-14b-raw.gif compared to the 20G http://lovehorsepower.com/MR2_Docs/compressor_flow_maps.htm (first map) you can see that the 20G can flow up to 640CFM at a pressure ratio of 2, while the 14B can flow only around 420CFM (0.2m^3/s) and I believe these are actually "corrected" airflows so they are actually mass flows, with density taken into account.

Since we know that the impeller on the 20g is larger diameter than the 14B, that means that if both are spinning at the same speed. The outer tips of the 20g will be moving faster, so the air coming off of them will be moving faster as well.
The compressor is able to increase the pressure by slowing down the air in the casing. The only thing I can assume then is that in the 20g, the air is coming off much faster, which allows it to be slowed down slightly to increase the pressure by 15psi say. Meanwhile on the 14B the flow would have to be slowed down by the same amount, which then has it end up at a lower velocity, and therefore lower flow rate.

Of course this whole scenario assumes that they're rotating at the same speed, which could be totally incorrect what with the compressor being powered off the exhaust...

Ok since I didn't want to talk out of my ass too much i decided to do some quick calculations on the temperature rise.

First of all, there will be an inherent temperature rise from increasing the pressure. This temperature rise is dependant solely on the pressure ratio, P2/P1 which for my example will be 2, corresponding to a manifold pressure of 15psi. This will determine the temperature ratio. So regardless of what kind of compressor you are using, the temperature will increase by this amount.

Assuming air comes in at 25C, or 298K the air will exit at 363K or 90C. Now if we take efficiencies into account we'll have to look at the power that is imparted into the fluid. This is simplified as pressure rise times flow rate. Using a flow rate of 0.2m^3/s and a pressure rise of 15psi (103kpa) I calculated power to be 20.7kW into the fluid. At this example, the 14B is running at 60% efficiency, and the 20G is running at 77% efficiency. This means that there will be 6.2kW of heat added by the 20g and 13.8kw added by the 14B. Dividing by specific heat (1.005kJ/kgK) and mass flow rate (0.2m^3/s*1.2kg/m^3) we get that the temperature rise for the 20g is 25.6C and the temperature rise for the 14B is 57C.

So perhaps i wasn't exactly correct in my assumption that temperature wasn't that different. However the final temperatures for the two compressors are 115C and 147C. Which corresponds to densities of 0.909kg/m^3 and 0.835kg/m^3 and these values are not that far off when it comes down to it.

That's almost 9% more air getting to the engine...

I guess that is a pretty significant increase when it comes down to it. However after cooling the temperatures would be closer still since the hot air would be cooled slightly more than the cool air would.

I guess it could be pure efficiency that causes the improvement, but I still think there's more to it than that. But I guess without putting pressure and temperature sensors throughout the intake, it would be impossible to tell.

I think that's the bottom line. Unless pressures, temperatures, etc are known fairly precisely and over very short time frames, in many different areas of the intake, etc, it's largely guesswork.

It's probably why most engine builders, etc would probably just rely on observed results and not worry a great deal on the entire explanation in all it's complicated minutia. 'Course it does still make for an interesting discussion. Certainly there's no harm (or a waste of time) in attempting to understand the physics behind it all.

Thanks for doing the math, Rob
At least now the question is answered pretty well: Yes, a bigger turbo @ same boost = more air and power.

I also agree there is likely more going on and it would be difficult to prove.
Mainly the boost gauge is not very accurate and I think that is a big problem for this discussion. Or maybe not, who knows!

Here is a dyno graph I found showing max power gains after changing only the turbo..

Yeah, I guess that's mostly the way everything works. Test it in real life don't worry about the theory, usually theory isn't entirely accurate anyway...

But thinking about this stuff is all super interesting to me. Attempting to apply my schooling to something interesting. Although I had to laugh at myself for being home doing math for entertainment on a Friday night...

LOL

That's awesome man. Feels good to apply that knowledge to something in the real world though doesn't it? Instead of just doing questions from a stupid text book.

Oh, I still haven't had a chance to read the thread fully yet.

WHY SO BUSY!?